fix: mkStructView

Reamrk: `1.2` is a numLit

src/Init/Lean/Elab/StructInst.lean

@@ -298,25 +298,37 @@ def Field.toSyntax : Field Struct → Syntax
     stx
   | _ => unreachable!
 
-private def toFieldLHS (stx : Syntax) : FieldLHS :=
-if stx.getKind == `Lean.Parser.Term.structInstArrayRef then FieldLHS.modifyOp stx (stx.getArg 1)
+private def toFieldLHS (stx : Syntax) : Except String (List FieldLHS) :=
+if stx.getKind == `Lean.Parser.Term.structInstArrayRef then
+  pure $ [FieldLHS.modifyOp stx (stx.getArg 1)]
 else
   -- Note that the representation of the first field is different.
   let stx := if stx.getKind == nullKind then stx.getArg 1 else stx;
-  if stx.isIdent then FieldLHS.fieldName stx stx.getId
+  if stx.isIdent then pure $ [FieldLHS.fieldName stx stx.getId]
   else match stx.isNatLit? with
-    | some idx => FieldLHS.fieldIndex stx idx
-    | none     => unreachable!
+    | some idx => pure $ [FieldLHS.fieldIndex stx idx]
+    | none     => match stx.isLit? numLitKind with
+      | some val =>
+        let parts := val.split $ fun c => c == '.';
+        parts.mapM $ fun part =>
+          match Syntax.decodeNatLitVal part with
+          | some idx => pure $ FieldLHS.fieldIndex (mkStxNumLit (toString idx) stx.getHeadInfo) idx
+          | none     => throw "unexpected structure syntax"
+      | none     => throw "unexpected structure syntax"
 
-private def mkStructView (stx : Syntax) (structName : Name) (source : Source) : Struct :=
+private def mkStructView (stx : Syntax) (structName : Name) (source : Source) : Except String Struct := do
 let args      := (stx.getArg 2).getArgs;
 let fieldsStx := args.filter $ fun arg => arg.getKind == `Lean.Parser.Term.structInstField;
-let fields := fieldsStx.toList.map $ fun fieldStx =>
+fields ← fieldsStx.toList.mapM $ fun fieldStx => do {
   let val   := fieldStx.getArg 3;
-  let first := toFieldLHS (fieldStx.getArg 0);
-  let rest  := (fieldStx.getArg 1).getArgs.toList.map $ toFieldLHS;
-  ({ref := fieldStx, lhs := first :: rest, val := FieldVal.term val } : Field Struct);
-⟨stx, structName, fields, source⟩
+  lhs ← toFieldLHS (fieldStx.getArg 0) | throw "unexpected structure syntax";
+  lhs ←
+    (fieldStx.getArg 1).getArgs.toList.foldlM
+      (fun lhs lhsStx => do lhsNew ← toFieldLHS lhsStx; pure (lhs ++ lhsNew))
+      lhs;
+  pure $ ({ref := fieldStx, lhs := lhs, val := FieldVal.term val } : Field Struct)
+};
+pure ⟨stx, structName, fields, source⟩
 
 def Struct.modifyFieldsM {m : Type → Type} [Monad m] (s : Struct) (f : Fields → m Fields) : m Struct :=
 match s with
@@ -807,11 +819,14 @@ structName ← getStructName stx expectedType? source;
 env ← getEnv;
 unless (isStructureLike env structName) $
   throwError stx ("invalid {...} notation, '" ++ structName ++ "' is not a structure");
-struct ← expandStruct $ mkStructView stx structName source;
-trace `Elab.struct stx $ fun _ => toString struct;
-(r, struct) ← elabStruct struct expectedType?;
-DefaultFields.propagate struct;
-pure r
+match mkStructView stx structName source with
+| Except.error ex  => throwError stx ex
+| Except.ok struct => do
+  struct ← expandStruct struct;
+  trace `Elab.struct stx $ fun _ => toString struct;
+  (r, struct) ← elabStruct struct expectedType?;
+  DefaultFields.propagate struct;
+  pure r
 
 @[builtinTermElab structInst] def elabStructInst : TermElab :=
 fun stx expectedType? => do

src/Init/LeanInit.lean

@@ -547,7 +547,7 @@ private partial def decodeDecimalLitAux (s : String) : String.Pos → Nat → Op
     if '0' ≤ c && c ≤ '9' then decodeDecimalLitAux (s.next i) (10*val + c.toNat - '0'.toNat)
     else none
 
-private def decodeNatLitVal (s : String) : Option Nat :=
+def decodeNatLitVal (s : String) : Option Nat :=
 let len := s.length;
 if len == 0 then none
 else

tests/lean/run/structInst4.lean

@@ -20,3 +20,6 @@ def foo : Foo := {}
 #check { x[1].2 := true, .. foo }
 #check { x[1].fst.snd := 1, .. foo }
 #check { x[1].1.fst := 1, .. foo }
+
+#check { x[1].1.1 := 5, .. foo }
+#check { x[1].1.2 := 5, .. foo }